Diffusion MRI – looking at water moving
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Dr Richard Watts from the Department of Physics and Astronomy at the University of Canterbury talks about a technique called diffusion MRI that looks at how water molecules move around the brain.
One clinical application of diffusion MRI is to look at the amount of water in brain cells after a patient has suffered a stroke. In the early stages of a stroke, brain cells swell with water. Later on, the cells start to die off and shrivel up. Not only can you see this on an MRI, but it is possible to determine how recently the stroke must have occurred.
DR RICHARD WATTS
A diffusion MRI looks at how water molecules move around within the brain. It’s just another version of MRI, but we can make MRI sensitive to different properties. The way water moves around in the brain depends on what structures there are there, so it depends on the cells that are within the brain. So it’s actually a very sensitive measure of what’s happening on a cellular level, on a microscopic level. So we can see these microscopic changes which are very subtle, you would never see them if you were just looking at a large image of the brain at a kind of millimetre resolution, we’re looking at micron-scale damage and we can see those things happening. And these techniques that we’re developing are very sensitive to these very subtle, but important changes and these changes you can correlate them with symptoms that people develop later.
A clinical example of using diffusion MRI is when somebody has a stroke, and when you have a stroke essentially one of the blood vessels has been blocked, and so you’re not delivering blood to the cells in the brain. So what happens initially is that these cells contain a lot of salt and a lot of sodium and normally when they’re working properly they have these pumps, which pump water and salt in and out of the cells. If you cut off the blood supply initially they can’t do this anymore, so what happens is that the cells swell up. Suddenly a lot of water goes into the cells. So you’ve now got a whole lot of water within the cells, and not much water outside the cells. Water inside the cells isn’t very free to move around. It has low diffusion. So we see this on our diffusion MRI Scan. In the early stages of a stroke, the cells are swelling up, and there’s a reduced amount of diffusion of the water within the cells because you have a cell wall there. Now later on in the stroke the cell’s will start to die off, and when they die off they shrivel away to nothing. And then you’ve got a lot of free water there, a lot of water without these structures there. And suddenly the diffusion is in the opposite direction, you’ve got lots of diffusion, the water is free to move around. So not only can you see on an MRI Scan that somebody’s had a stroke, but you can actually stage it as well. You can see if it’s a stroke that’s happened recently or is it something that happened three months ago.